Long used to rejuvenate roads in the Southwest, asphalt rubber is gaining fans across North America.

Engineers and chemists began exploring ways to add rubber to asphalt in the 1920s and came up with several processes to produce what's known generically as rubberized asphalt. In the “wet process,” crumb rubber (shown) is blended at high temperatures with liquid asphalt and other binders. “Asphalt rubber” refers to a specific material containing at least 15% ground rubber as defined in ASTM D6114-97 (2002). Photos: Rubber Pavements Association

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Mix temperatures for asphalt rubber are slightly higher than conventional hot mixes, and workability is limited. The material should be at least 350° F for placement and fully finished before the temperature gets below 275° F. Such requirements mean that steel-wheeled rollers must be used for compaction.

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In 2002, Arizona DOT placed 1 inch of asphalt rubber on a stretch of concrete pavement that carries 150,000 vehicles/ day. The resulting reduction in traffic noise—averaging 3 to 5 decibels—was so striking that the public clamored for more. By the end of that year, the department announced it would install the overlay on all 115 miles of the state's freeways.

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At least one state has figured out how to preserve deteriorated pavement to provide a smooth, safe, long-lasting surface that reduces traffic noise and tire wear—all while putting millions of scrap tires to good use.

It sounds almost too good to be true, but proponents say asphalt rubber allows agencies to do just that. And judging from the Arizona DOT's (ADOT) more than 20 years of experience overlaying asphalt rubber on concrete and asphalt pavements, those proponents are right.

One of the main advantages of an asphalt rubber friction course, or ARFC, is its thinness. Just 1 inch needs to be applied over portland cement concrete pavement, and just 1/2 inch is needed over existing asphalt.

Used in a friction course, asphalt rubber costs about $2.15/square yard, compared to $1.55/square yard for conventional mixes. A durable, smooth-riding surface can be restored without changing the basic road geometry, drainage pattern, or other factors such as curb and gutter configuration.

ADOT is considered the pioneer of using this budget-stretching technology to preserve and prolong pavement life.

In 1964, Phoenix began using rubberized asphalt and gravel as a chip seal for city streets. Though successful in preserving the pavement,

the program was discontinued in 1989 because of potential damage to cars by loose gravel. So the city began using an asphalt rubber hot mix to add a 1-inch overlay to streets. The material exhibited several advantages:

It didn't reflect cracks from the existing pavement.

It proved more durable and skid-resistant than conventional asphalt.

It reduced traffic noise to provide a smoother, quieter ride.

The Federal Highway Administration (FHWA) developed a mix design procedure for open-graded friction courses and began promoting their use to improve the friction resistance of asphalt pavements in the mid-1970s. These overlays also were designed to drain quickly in rainy weather and improve safety by increasing traction and reducing water spray that impedes driver visibility.

Many state DOTs used the FHWA procedure, but with varying results. Several states, particularly those that used conventional asphalt binders, experienced performance problems such as premature raveling of the surface, debonding, and stripping in underlying layers, and they gave up on open grades. Although FHWA revised the mix design procedure in 1990, most states with previously unsatisfactory experiences didn't resume their use.

In Arizona, on the other hand, asphalt rubber was used as the binder for friction courses, producing a much more durable product. When highway officials became aware of the reduction in tire noise, they began researching that area as well.

During the 1990s, Phoenix resurfaced more than 200 miles of streets with 450,000 tons of rubberized asphalt created from about 1.1 million old tires. The state DOT has used more than 4.2 million tons since 1988, recycling about 15 million old tires in the process.